Just picked this up a few days ago and am dying to know just what it is.

The numbers stamped on the frame and engine match, and correspond to a '59 model but I don't know the engine size. I assume it's either a 500 or a 650. There are no letters to ID the engine - it's just a 6 digit number.
So my question is, for any of you old Triumph cats out there : Is there any other way to determine engine size w/out actually disassembling and measuring bore, stroke, etc ?

Most likely this engine is going to need to be torn down anyway as this bike is a rustbucket. I'm not even sure if the engine is free, since I can't seem to find how to turn it over (The kickstart doesn't engage)
Isn't there a way to turn the crankshaft externally?
Looks like I would need to remove the whole sidecover.

there should be a model designation stamp on the engine, like 6T or T120, TR6 or 5T.
take a pic of the number side of the engine and post it on here so we can tell what has happened.
As for turning the engine over just put it in gear and roll it.

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I don't see any model stamp w/letters like that.
There is just a 6 digit number on the left side of the cases just under the cylinder base.
Should I look somewhere else?

Putting it gear and turning the wheel won't work, the clutch or something is messed up so it won't go in any gear. I know I'll have to tear this thing all apart anyway but it would be nice to know if the engine turns at least.

that's where the numbers are, there should be a model stamped into it. it is common to see engine numbers that are incorrectly re-stamped in replacement cases.
and you can't turn the engine with a kicker or a wheel without a clutch.

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It is not a '59 as the duplex frame didn't happen until '60 and that one appears to have the upper brace so it is '61 or '62. It is a TR6 trophy 650, no 500 engines were pre unit at that time, they had all become unit by then.
Definitely set up as a desrt sled. Due to the common practice of blowing stuff up and just re-stamping another engine to match the frame on race bikes it has been done like that. Who knows for sure what the engine is. That number you show on the frame is a casting number and is on all of them so it means nothing except that the neck casting is for a late duplex frame. The alternator primaries are also correct for a late pre-unit as the generator ceased to be by then and your case have no provision for one. A '59 would have had one.
Robbie

Being a race bike, the odds of the engine being original to the frame are smaller. The engine may even be built from pieces of different motors. The cases are alternator cases. (5T '55-'58, 6T '54-'62, T110 '60-61, TR6 or T120 '60-'62) If you've got the right books you can tell more by the casting numbers. The stamped number, however, is not original to that engine's left case half.

The frame's serial number should be stamped in the left side of the neck and be either 029364-030424 or D101-D25000 (or thereabouts. I don't know the final duplex frame number so that's a guess based on previous years production) Your number is a '59 number, but may have been stamped into that late '60-'62 frame to match a '59 title that someone had long ago. It was a common practice back when, both for race bikes that kept blowing parts and bending frames, and unfortunately for stolen bikes. It's a nice bunch of parts you've got there though. Find yourself a titled pre unit frame and a clean set of cases and build a bike. Or alternatively, you could part that bike out and scrap the badly numbered pieces.

Here's a pic of what a duplex frame serial number should look like.

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" Find yourself a titled pre unit frame and a clean set of cases and build a bike. Or alternatively, you could part that bike out and scrap the badly numbered pieces. "

Oh no, this bike is getting restored. Here in AZ it's easy to get a title, I can just take the bike down to Motor Vehicle Dept, they will inspect it and I can get a bonded title. Done it many times. They will probably ID this as a '59 since that's what the numbers match to.

Here's my '73 Guzzi that looked not much better than the Triumph when I first brought it home :

I can just take the bike down to Motor Vehicle Dept, they will inspect it and I can get a bonded title.

It would be best to title it as a '61 or '62 as none of those frames existed until then. Are you sure there is no frame number?
What is the transmission number? It is on top behind that clutch arm.
As I said, I believe it is a 650. No sense in using a 500 engine in that bike. A 5T would have had a cast iron head ( although the cases could have been used with 650 internals and top end). A TR5 or T100 would have generator cases. What does the plate on the timing cover say? If it is original to the engine it will tell you the model!
And those cases appear to be unmolested under the number stamping so were likely a replacement set. Whoever stamped them left half the number off though, as it should have the model designation on it!

Yes, the s/n 021939 on the engine is also on the frame.
I don't think I will have a say in what year the bike gets titled as but I will try.
The timing cover plate just says " made under patent numbers (series of numbers)
The transmission s/n is 129477.
I had a friend look at it and he says the transmission is from a 650, and with the comments, I believe I do indeed have a TR6 650 here.
Appreciate the help !

I wouldn't think this would be the bike to "retore", as in original, too obviously bad numbers that will only result in bad papers when titled. Might be a better bobber or bitsa. The only link to a TR6 (trophy) would be the alloy head, which was also on the T110. There's really no way of knowing what model that lower end originally was. You could look for seat hinge mounts on the rear frame section, 6T and T110 had hinged seats, TR6 and T120 did not. These would probably be cut off by now, but there may be a bump to feel. Sport models ( TR6 & T120) had a smaller tank as well, but most racers would want the smaller type so that's no help.

When I say ' restore " I mean go through the whole bike mechanically, and also make it look good but I'm not into full cosmetics, I like a bike that is reliable, PERFORMS, and looks pretty good... Form follows function !